This invention relates to a process for separating synthesis gas (xe2x80x9csyngasxe2x80x9d) containing hydrogen, steam, CO2, and CO into fuel cell quality hydrogen and xe2x80x9csequestration readyxe2x80x9d carbon dioxide.
Global warming due to CO2 and other Greenhouse gases is one of the major problems facing modern industrial society. One major source of CO2 emissions to the atmosphere is the combustion of fossil fuels in power plants producing electricity. The problem of CO2 emissions from fossil fuels could be significantly reduced, if not eliminated, by converting the fuel to a mixture containing H2, CO2, CO and water vapor, (syngas), separating the mixture into two streams, one containing hydrogen and the other containing CO2, and then isolating the CO2 and using the hydrogen to produce electricity in PEM fuel cells.
Each of the separated streams of gas must meet the purity requirements for which it is intended. Thus, hydrogen to be used in a PEM fuel cell must be free of substances that tend to xe2x80x9cpoisonxe2x80x9d the fuel cell, i.e., the hydrogen must have a CO content of less than a few ppm. The H2S content must also be very small. Since gases like CO2 and CH4 are not catalyst poisons, larger amounts can be tolerated. Isolating CO2 normally requires compressing and cooling the gas into a liquid. Although some impurities such as SO2 will readily liquefy along with the CO2 and hence do not interfere with sequestration, others such as CO and CH4 are not readily liquefied and interfere with the process if present in excessive amounts.
The use of CaO to remove CO2 from gas streams in general, and specifically from syngas, has been described in the literature. A reference by Han and Harrison (Chemical Engineering Science, 49, 5875-5883, 1994) is typical of such prior art and describes a process in which a mixture containing H2, steam, CO and CO2 is passed through a bed of calcined limestone. CO2 is removed by the reaction with the calcined limestone, i.e., CaO+CO2 CaCO3. Since calcined limestone serves as a catalyst for the water gas shift reaction, the CO and CO2 are in mutual equilibrium via the reaction CO+H2O CO2+H2, and removal of the CO2 also removes the CO. Thus, hydrogen gas was purified in the Hann and Harrison
This 1994 reference is subject, however, to an important limitation. Although the process of purifying the hydrogen as described converts the CaO to CaCO3, the article is silent with respect to the regeneration of the CaCO3 back into CaO. Even though it may be within the skill of the art to recalcine the CaCO3 back to CaO, the process has obvious disadvantages. Converting CaCO3 to CaO is a strongly endothermic process, requiring a large input of heat energy. Typically, the purification of hydrogen is done using a packed bed of CaO. When heat is put into a packed bed by heating the walls of the bed, the sections of the bed closest to the walls tend to insulate the interior portions of the bed. Thus, once the bed is converted to CaCO3, reconverting it to CaO by heating the walls of bed involves a very awkward and inefficient heat transfer situation.
In U.S. Pat. No. 5,339,754 and related U.S. Pat. Nos. 5,509,362 and 5,827,496 (incorporated herein by reference), a new method of burning fuels is disclosed using catalyst materials. The ""362 teaches the use of a metal oxide catalyst that can be readily reduced. Similarly, in the ""496 patent, the catalyst consists of a material that can be readily reduced when in an oxidized state and readily oxidized when in a reduced state. The fuel and air are alternately contacted with the metal oxide. The fuel reduces the metal oxide and is oxidized to CO2 and water vapor. The air re-oxidizes the catalyst and becomes depleted of oxygen. Thus, combustion can be effected without the need for mixing the fuel and air prior to or during the combustion process. If means are provided whereby the CO2 and water vapor and the oxygen depleted air can be directed in different directions as they leave the combustion process, then mixing can be completely avoided.
This new method of combustion is now generally referred to in the art as xe2x80x9cunmixed combustion.xe2x80x9d The ""754 patent discloses various metal oxides that can be readily reduced, including oxides of silver, copper, iron, cobalt, nickel, tungsten, manganese, molybdenum and mixtures thereof, supported on alumina. The ""496 patent discloses that the readily reduced metal oxides are selected from a group consisting of nickel/nickel oxide, silver/silver oxide, copper/copper oxide, cobalt/cobalt oxide, tungsten/tungsten oxide, manganese/manganese oxide, molybdenum/molybdenum oxide, strontium sulfide/strontium sulfate, and barium sulfide/barium sulfate.
One embodiment of the ""362 patent also teaches a process for steam reforming of hydrocarbons. In this process, the reaction between the hydrocarbon and steam is carried out over a nickel catalyst in the presence of CaO. While this steam reforming reaction is endothermic, it produces CO2 which reacts exothermically with the CaO, making the overall reaction weakly exothermic. Thus, the need to supply the heat consumed by the steam reforming reaction by putting heat in through the reactor walls can be avoided. Eventually, the CaO is largely converted to CaCO3. When this occurs, the production of hydrogen by steam reforming is halted. Air is passed through the reactor, oxidizing the nickel catalyst to nickel oxide. The ratio of nickel catalyst to CaO/CaCO3 is chosen so that the oxidation of the nickel catalyst liberates enough heat to decompose the CaCO3 back to CaO. When hydrocarbon and steam are again fed through the reactor, the NiO is reduced to Ni and the production of hydrogen begins again.
Another embodiment of the ""362 patent, relating specifically to coal combustion, is discussed in Paper 98F-36 by R. K. Lyon and J. A. Cole at the 26 and 27 October 1998 meeting of the Western States Section of the Combustion Institute. This paper discloses a process in which coal is oxidized in a fluid bed by SO2 in the presence of Fe2O3. The reaction between the coal and the SO2 reduces it to elemental sulfur and other reduced sulfur species that are oxidized back to SO2 by the Fe2O3. thus, the SO2 acts as a catalyst, facilitating the oxidation of the coal by the Fe2O3. In this process, the Fe2O3 is reduced to FeO, which is then reoxidized back to Fe2O3 in the presence of air.
For situations in which the fuel contains sulfur, the Lyon and Cole reference teaches the existence of a xe2x80x9cthresholdxe2x80x9d amount of Fe2O2. If the conversion of the Fe2O3 to FeO is allowed to exceed this threshold, the sulfur in the fuel forms FeS during the coal oxidation step. During the subsequent reoxidation with air of the FeO back to Fe2O3, the FeS is oxidized to Fe2O3 and SO2 and emitted to the atmosphere. Keeping the Fe2O3 conversion below the threshold prevents SO2 emissions.
While the Western States paper generally discloses the existence of a xe2x80x9cthreshold,xe2x80x9d the reference does not state what the threshold is, i.e., it does not teach or otherwise quantify the amount of Fe2O3 that can be reduced to FeO without exceeding the xe2x80x9cthreshold.xe2x80x9d
Taken together, the above references reflect a number of limitations in the art. The Combustion Institute paper teaches a method of producing sequestration ready CO2, but does not disclose any method for producing fuel cell quality hydrogen. Nor does the paper teach any method of gas separation. The embodiment of U.S. Pat. No. 5,509,362 (which relates to steam reforming) teaches the production of hydrogen, but not the separation of mixtures containing hydrogen or the production of sequestration ready CO2. The Hann and Harrison reference teaches the separation of hydrogen from gas mixtures, but not the production of sequestration ready CO2.
Thus, a clear need exists in the art for a new and more efficient method of using syngas whereby gas mixtures containing hydrogen, steam, CO2 and CO can be separated into fuel cell quality hydrogen and sequestration ready CO2.
The present invention provides a new energy efficient method of separating gas mixtures containing hydrogen, steam, CO2 and CO (syngas) into fuel cell quality hydrogen and sequestration ready CO2. The method according to the invention also allows sulfur compounds present in small amounts in the syngas feed to leave as SO2 in the sequestration ready CO2.
Specifically, the present invention concerns a new and improved method of separating gas mixtures containing synthesis gas into separate streams of wet hydrogen containing greatly reduced amounts of CO2 and less than 50 ppm CO, and of sequestration ready CO2 containing less than 1% fixed gases, i.e., less than 1% gases which at ambient temperature do not readily liquefy under pressure.
In one exemplary embodiment, a mixture of limestone and iron oxide circulates between two fluidized beds. One bed is fluidized with a gas containing H2, steam, CO2 and CO, while the other bed is fluidized with a gas containing steam and oxygen. As the fluidizing gas containing hydrogen, steam, CO2, and CO, or syngas, passes through the bed, the CO2 reacts with CaO, forming CaCO3. Virtually all of the CO is removed by a combination of two reactions. Some CO is removed by the water gas shift reaction, forming hydrogen and CO2 with the CO2 being removed by reaction with the CaO. The rest of the CO is removed by reaction with the iron oxide, reducing Fe2O3 to FeO, a process that also forms CO2, which is removed by reaction with the CaO.
Some of the hydrogen is also removed by reaction with the iron oxide, reducing Fe2O3 to FeO, while the remainder of the hydrogen passes through the fluid bed, leaving in a purified state for use in, for example, a PEM fuel cell. The circulation of solids between the two beds carries the CaCO3 and FeO, along with unreacted CaO and Fe2O3, to the second fluid bed where the FeO is oxidized to Fe2O3, liberating heat that decomposes the CaCO3 back to CaO.
Sulfur compounds in the syngas typically are present as H2S. H2S reacts with FeO and CaO to form FeS and CaS, respectively. When the solids circulation carries FeS and CaS to the second fluid bed, they are oxidized, liberating SO2 and forming Fe2O3 and CaO, respectively.
Accordingly, in its broadest aspects, the present invention covers a process for separating syngas into fuel cell quality hydrogen and sequestration ready carbon dioxide, comprising the steps of (1) circulating a mixture of solids comprising calcium compounds present as CaO, Ca(OH)2 and CaCO3 and mixtures thereof, and iron compounds present as FeO, Fe2O3, Fe3O4 and mixtures thereof, between first and second fluidized bed reactors; charging the first fluidized bed reactor with a first gas mixture containing high temperature steam, H2, CO2, H2S and CO; (2) charging the second fluidized bed reactor with a second gas mixture containing high temperature steam and oxygen; (3) reacting CO2 present in the first gas mixture with Cao present in the first fluidized bed reactor to form CaCO3; (4) reacting a portion of CO present in the first gas mixture with steam in a water gas shift reaction to form hydrogen and CO2 and reacting the remaining portion of CO present in the first gas mixture with iron oxide to form FeO and CO2; (5) reacting any H2S or other sulfur containing species in the first gas mixture with CaO to form CaS and H2O; (6) circulating CaCO3, CaS and FeO formed in the first fluidized bed reactor to the second fluidized bed reactor; (7) oxidizing FeO and CaS present in the second fluidized bed reactor to form Fe2O3, CaO and SO2; (8) removing substantially pure hydrogen from the first fluid bed reactor; and finally (9) removing CO2, high temperature steam and SO2 from the second fluid bed reactor.
Another aspect of the present invention concerns a new apparatus for separating syngas into fuel cell quality hydrogen and sequestration ready carbon dioxide, comprising first and second fluidized bed reactors containing solids containing calcium compounds present as CaO, Ca(OH)2 and CaCO3 and iron compounds present as FeO, Fe2O3 and Fe3O4; first and second solids transfer lines for circulating said solids mixtures between said first and second fluidized bed reactors; a first inlet feed for charging said first fluidized bed reactor with high temperature steam and syngas; a second inlet feed for charging said second fluidized bed reactor with high temperature steam and oxygen; means for removing substantially pure hydrogen from said first fluidized bed reactor.; means for removing high temperature steam, CO2 and SO2 from said second fluidized bed reactor.